In copending application RD-13036 there is taught a method of making aromatic bis(ether phthalic anhydride) by effecting reaction between molten aromatic bis(ether N-organophthalimide) and an aqueous phthalic acid solution containing an imide-anhydride exchange catalyst, for example, a triorganoamine. Upon reaching equilibrium, the exchange reaction mixture is extracted with an organic solvent to effect the removal of unreacted aromatic bis(ether N-organophthalimide) and N-organophthalimide.
Experience has shown that when the imideanhydride exchange mixture has been extracted several times to provide for aromatic bis(ether phthalic anhydride) having acceptable purity, the concentration of the triorganoamine exchange catalyst continues to build-up in the recycled organic solvent and cause a significant catalyst loss and eventually interferes with the recovery of the aromatic bis(ether phthalic anhydride).
The present invention is based on the discovery that the build-up in concentration of the triorganoamine catalyst in the organic solvent used to extract unreacted aromatic bis imide and N-organophthalimide reaction product can be controlled. The triorganoamine catalyst recovery from the organic solvent can be achieved if the extracting organic solvent is itself extracted with an aqueous phthalic acid solution before it is recycled. Experience has shown that treating the extracting organic solvent having 1-20% by weight of triorganoamine with 20-35% by weight of aqueous phthalic acid solution removes 85-95% triorganoamine from the organic solvent. The resulting extracted organic solvent after subsequent processing to effect imide removal, can then be recycled to the extraction column. The aqueous mixture of phthalic acid and triorganoamine catalyst containing additional triorganoamine extracted from the organic solvent can be recycled to the imide-anhydride exchange reactor.